Low salinity surface water inhibits local deepwater formation in the modern North Pacific.
Instead, southern-sourced Circumpolar Deep Water (CDW) fills the basin, which is the product of water
masses formed from cold sinking centers in the Southern Ocean and North Atlantic. This CDW is responsible
for transporting a significant amount of global heat and dissolved carbon in the deep Pacific Ocean. The
history of its flow and the broader overturning circulation are widely assumed to be sensitive to climate
perturbations. However, insufficient records exist of CDW presence in the deep North Pacific with which to
evaluate its evolution and role in major climate transitions of the past 23 Ma. Here we report sedimentary
coatings and fish teeth neodymium isotope values—tracers for water-mass mixing—from deepwater
International Ocean Discovery Program Site U1438 (4.7 km water depth) in the Philippine Sea, northwest
Pacific Ocean. Our results indicate the water mass shifted from a North Pacific source in the early Miocene to a
southern source by ~14 Ma. Within the age model and temporal constraints, this major reorganization of
North Pacific water mass structure may have coincided with ice sheet build up on Antarctica and is most
consistent with an increased northward flux ofCDWdue to enhanced sinking of cold water forced by Antarctic
cooling. The northward extent of this flux may have remained relatively constant during much of the past 14Ma